1. Field of the Invention
The present invention relates to the measurement of blood pressure. More particularly, the present invention relates to an apparatus and method for measuring blood pressure using linearly varying air pressure.
2. Description of the Related Art
Blood pressure is the most useful, easy-to-measure index among a variety of indices of the health of a human being. Blood pressure is used as an index for diagnosing disorders of the circulatory system including the heart and blood vessels. Additionally, when blood pressure is above a normal range, particular medical treatments may be required.
Arterial blood pressure fluctuates due to the beating of the heart. The blood pressure at a period of time during which blood is discharged into the artery due to contraction of the ventricle of the heart is called the xe2x80x9csystolic blood pressure.xe2x80x9d The blood pressure at a period of time during which no blood is discharged into the artery due to extension of the ventricle, but where blood pressure is maintained above zero due to the elasticity of the arterial blood vessel, is called the xe2x80x9cdiastolic blood pressure.xe2x80x9d
Blood pressure varies with many factors. Accordingly, it is difficult to measure blood pressure accurately through only a single measurement. For example, when a blood pressure measurement is taken in a hospital, false hypertension often occurs due to the stress of a patient and results in an accurate reading.
Blood pressure that is measured in the morning, before eating, is referred to as xe2x80x9cbasal blood pressure.xe2x80x9d Basal blood pressure is critical for diagnostic purposes, but is difficult to accurately measure under ideal conditions. In order to measure the basal blood pressure as accurately as possible, under near ideal conditions, blood pressure must be measured at home. To facilitate the measurement of basal blood pressure, there is an increasing need for an electronic blood pressure monitor that enables a person to measure blood at home easily.
Research has been conducted in a variety of aspects to enable people to accurately measure their blood pressure at home. In particular, due to the advances in the electronics industry, automated blood pressure monitors, with which human blood pressure can be indirectly measured, have become available. One automated blood pressure monitor in current use is based on a volume oscillometric method that requires no particular transducer or microphone.
It is assumed that there is a predetermined characteristic ratio between the cuff oscillation magnitudes during systolic and diastolic cycles with respect to the maximum oscillation magnitude of the cuff. In a conventional blood pressure measuring method using an arterial blood pressure pulsation measuring apparatus the blood pressure measured under the cuff oscillation condition inducing about 50% of the maximum cuff oscillation magnitude is estimated as the xe2x80x9csystolic blood pressure,xe2x80x9d and the blood pressure measured under the cuff oscillation condition inducing about 50-80% of the maximum cuff oscillation magnitude is estimated as the xe2x80x9cdiastolic blood pressure.xe2x80x9d These conclusions were obtained through trials conducted on human beings and dogs.
FIG. 1 is a graph showing a pressure signal of the cuff and a volume oscillometric signal, in which the pressure signal is denoted by the dashed line and the volume oscillometric signal is denoted by the solid line.
The pressure signal of FIG. 1 appears as the pressure of the cuff is linearly reduced. The volume oscillometric signal of FIG. 1 is obtained by passing the pressure signal through a 0.5 Hz high-pass filter and amplifying the filtered pressure signal. As the pressure of the cuff is reduced at a constant rate, the maximum amplitude 10 of the volume oscillometric signal occurs at a cuff pressure for the average blood pressure. The systole amplitude 12 corresponding to 50% of the maximum amplitude 10 is read as the systolic blood pressure. The diastole amplitude 14 corresponding to 75% of the maximum amplitude 10 is read as the diastolic blood pressure. The ratios of the systole amplitude 12 and the diastole amplitude 14 to the maximum amplitude 10 are referred to as the xe2x80x9ccharacteristic ratios.xe2x80x9d These characteristic ratios have a variation of 10-20% depending on the individual being measured and are affected considerably by the appearance and elasticity of the cuff, the shape and amplitude of the arterial pressure waveform, and a variety of vital dynamical factors including the pressure transfer characteristics of the artery, the arm, and the cuff, and the viscoelastic characteristics of the arterial blood vessel.
Most blood pressure measuring apparatuses in current use measure blood pressure at the brachium (i.e., the upper arm). Accordingly, in preparation for a blood pressure measurement, an individual faces the inconvenience of taking off his/her jacket and rolling up a shirtsleeve. Additionally, a relatively high pressure is applied several times during the measurement of blood pressure, which may cause discomfort for the individual.
Air pressure valves typically have nonlinear exhaust characteristics. This is especially true in a finger type cuff having a small-capacity air bag, where the exhaust characteristics tend to be even more nonlinear. If the air pressure of the cuff is reduced nonlinearly, then the maximum oscillation is affected. Therefore, such a conventional blood pressure measuring apparatus is not able to measure blood pressure accurately.
In an effort, to solve the above-described problems, it is a first feature of an embodiment of the present invention to provide a blood pressure measuring apparatus capable of measuring blood pressure using linearly varying air pressure and which is conveniently applied to an individual having a blood pressure measurement taken.
It is a second feature of an embodiment of the present invention to provide a method for measuring blood pressure using linearly varying air pressure in the blood pressure measuring apparatus described above.
To provide the first feature of an embodiment of the present invention, there is provided an apparatus for measuring blood pressure using linearly varying air pressure, including: a compression unit to be applied to surround a predetermined site of an individual""s body, the compression unit being inflatable and deflatable; an air pump that injects air into the compression unit in response to a first control signal; a pressure sensor that senses the air pressure of the compression unit and outputs the result of the sensing; an analog-to-digital converter (ADC) that converts the result of the sensing from an analog form into a digital form and outputs the result of the conversion as a pressure signal; a controller that calculates a current pressure value of the compression unit from the pressure signal and a linear pressure of the compression unit that is linearly dropped corresponding to the calculated current pressure value, generates the first control signal in response to the current pressure value, and generates a second control signal from the result of a comparison of the current pressure value and the linear pressure to measure the blood pressure of the individual; a digital-to-analog converter that converts the second control signal into an analog form and outputs the result of the conversion as an exhaust control signal; and a proportional control valve that exhausts air from the compression unit in response to the exhaust control signal.
To provide the second feature of an embodiment of the present invention, there is provided a method for measuring an individual""s blood pressure using linearly varying air pressure including: (a) injecting air into a compression unit until a reference pressure value is reached; (b) calculating a current pressure value of the compression unit from a pressure signal output from an analog-to-digital converter (ADC), and a linear pressure for the current pressure value; (c) determining whether the current pressure value is greater than the linear pressure; (d) if the current pressure value is determined to be greater than the linear pressure, increasing a rate of dropping pressure of the compression unit; (e) if the current pressure value is determined to be equal to or less than the linear pressure, decreasing the rate of dropping pressure of the compression unit; (f) after (d) or (e), determining whether a number of current pressure values calculated is equal to fsxc3x97t, where fs denotes a sampling frequency of the analog-to-digital converter and t denotes a period of time required until a maximum pressure Pmax of the compression unit drops to a minimum pressure Pmin; (g) if the number of current pressure values is determined to be less than fsxc3x97t, then changing a variable i denoting the order of the pressure signal input from the analog-to-digital converter, and proceeding to (b); and (h) if the number of current pressure values is determined to be equal to fsxc3x97t, determining the blood pressure using the current pressure values in a quantity equal to fsxc3x97t.